Add mechanism for installing useful git hooks

Run install-git-hooks.sh to install all pre-commit hooks (in the
hooks subdirectory) into the git repository at the CWD.

An existing pre-commit script is not overwritten, but appended to.
A non-existent pre-commit script is created.
Already-installed hooks are not installed again.
Add a hook to reject SNAPSHOT dependencies

If your POM has a SNAPSHOT reference other than to the parent, it is
rejected. This is a simple yet effective check to ensure no SNAPSHOT
dependencies get accidentally introduced.